Organic photovoltaic component and method for production thereof

ABSTRACT

The invention relates to an organic photovoltaic component, in particular an organic solar cell, in which one or more layers is (are) structured.

The invention relates to an organic photovoltaic component, particularlyan organic solar cell.

Solar cells having the following cell structure, for example, are known:

Disposed on a substrate is a positive electrode (typically ITO, indiumtin oxide). On top of that is the hole-conducting layer, composed forexample of PEDOT with PSS as the anion. The next layer is an absorber,usually an organic semiconductor (e.g. a mixture of conjugated polymerwith fullerene). This is followed by the negative electrode (e.g. Ca/Agor LiF/Al). The individual layers can differ from this scheme, however,especially the electrodes, the conjugated polymer and also the acceptor(PCBM, a soluble methanofullerene).

Due to the very low mobility of the semiconductors typically used inthese solar cells, the active semiconductor layer (the absorber) is madevery thin (typically between 20 nm and 2000 nm) to preventrecombination. However, this thin absorber layer usually is notsufficient to fully absorb the incoming light. Some of the light istherefore lost (absorbed) at the back electrode or reflected there (andcoupled out again through the front of the solar cell).

The object of the invention is, therefore, to reduce these lossprocesses by means of a process step that is as simple and inexpensiveas possible.

The invention is directed to an organic photovoltaic componentcomprising a substrate, a positive electrode, an organic semiconductorand a negative electrode, wherein the substrate and/or one or moreadditional transport layer(s) between the electrode and thesemiconductor layer is (are) structured. The invention is also directedto a method for structuring the semiconductor layer of a photovoltaiccomponent by preserving an existing structure of a lower layer to whichthe semiconductor layer is applied.

In one embodiment of the invention, the substrate is structured and theelectrode and the semiconductor layer therefore follow the structuring,and the absorptivity of the semiconductor layer is thereby increased.

In another embodiment, the semiconductor layer is applied in such a waythat it planarizes the structure.

In one embodiment, plural layers beneath the semiconductor layer arestructured. Intermediate layers can also be built into the photovoltaiccomponent to create a structured surface to which the semiconductorlayer is applied.

Structuring one or more layers of the photovoltaic element improves thecoupling of light into the solar cell. This kind of structuring istherefore also known as “light trapping.”

The terms “organic material” and/or “functional polymer” hereinencompass all types of organic, metalorganic and/or organic/inorganicsynthetics, denoted in English, for example, by “plastics.” Thisincludes all types of materials except for the semiconductors that formconventional diodes (germanium, silicon) and typical metallicconductors. Hence, there is no intended limitation in the dogmatic senseto organic material as carbon-containing material, but rather, thebroadest use of silicones, for example, is also contemplated.Furthermore, the terms are not intended to be subject to any limitationwith respect to molecular size, particularly to polymeric and/oroligomeric materials, but instead the use of “small molecules” iscompletely feasible as well.

Light trapping is generally achieved by imparting a periodic structureto at least one of the layers of the solar cell. It has, in fact,already been proposed (M. Niggeman et al., “Trapping light in organicplastic solar cells with integrated diffraction gratings,” Proceedingsof the World Photovoltaic Congress, Munich 2001) to structure theabsorber periodically (for example by means of an embossing or stampingprocess). Embossing the semiconductor, however, is a critical processstep, since the sensitive semiconductor layer can easily become damagedduring this process. This notwithstanding, the structuring of thesemiconductor layer can be performed in the sense of the invention incombination with the structuring of the substrate and/or of anadditional transport layer.

The terms the upper layer “follows the structure” and/or “reproduces thestructure upwardly” merely describes [sic] the fact that at least someof the lower structure is traced upwardly, i.e., the lower structure isduplicated in part or in whole on top. The upper structure can alsoundergo additions to the structuring, so that a completely differentstructure is formed. The invention is not intended to be limited in anyway in this regard.

The invention is described in more detail below on the basis ofindividual examples relating to embodiments of the invention.

FIG. 1 shows a layer structure of a photovoltaic component in which thesubstrate is structured and is replanarized by an additional transportlayer, and the bottom electrode then already goes back to being appliedto a planar surface.

FIG. 2 shows a photovoltaic component in which an additional matchinglayer for adapting the optical properties is applied to the substrate insuch a way that the structure is reproduced upwardly and effects astructuring of the electrode layer, which is then planarized by ahole-conducting layer, so that the semiconductor layer is applied to aplanar surface.

FIG. 3 shows a photovoltaic component in which a bottom electrode isstructured on a planar substrate, the structure works its way through ahole-conducting layer, and finally the semiconductor layer is applied toa structured surface.

In FIG. 1, the substrate, identified as 1, can be a PET sheet or a layerof photoresist on glass. This substrate is structured and is coated withan additional layer 6, for example of a material having a highrefractive index, such as TiO₂, so that the structure is traced, and isthen replanarized by a layer 7 of a transparent material that can alsobe a PET sheet or a layer of photoresist on glass. The standard cell isthen processed on this substrate from the bottom up, as, first, a bottomelectrode 2, which is implemented as semitransparent (e.g. of ITO) forthe case in which the side on which substrate 1 is located is thelight-incident side of the photovoltaic component. Disposed thereon inthis embodiment is an additional organic electrode 3 a, for example ofPEDOT, and thereon the semiconductor layer 4 and a second electrode 3 band/or 5.

FIG. 2 illustrates a substrate 1 that is structured and to which isapplied a layer 6 of a material for example having a high refractiveindex, which follows the structure. Disposed thereon is the bottomelectrode 2, and on that an additional electrode or transport layer 3 athat planarizes the structure. The semiconductor layer 4 is applied to aplanar surface. The further structure includes an additional electrodeor transport layer 3 b and top electrode 5.

The material of layer 6 is generally a layer intended to provideimproved optical properties and/or optical matching, such as, forexample, a layer having a high refractive index.

FIG. 3 depicts a substrate 1 that is not structured, to which is applieda bottom electrode 2 that is structured, to which is applied anadditional layer 3 a that follows the structure, and to whose structuredsurface semiconductor layer 4 is applied. Semiconductor layer 4planarizes the structure, so that an additional electrode 3 b is appliedto a planar surface of semiconductor layer 4. A further electrode 3 band top electrode 5 are not structured in the illustrated embodiment.

For the case in which the bottom electrode is not on the light-incidentside, this electrode can also be made of a completely reflectivematerial.

The invention shows, for the first time, photovoltaic components whoseabsorptivity of light is increased by structuring one or more layers ofthe component, thereby improving coupling-in. The structuring of thelayers is performed without any mechanical or thermal stressing of thesemiconductor layer, which therefore remains undamaged.

The invention proposes, instead of structuring the semiconductor layer,which causes an increase in absorptivity but stresses the semiconductorlayer mechanically, chemically and/or physically, to structure thesubstrate before applying the positive or negative electrode and/or tostructure an organic transport layer (e.g. PEDOT) before applying thesemiconductor layer. The structuring steps involve the substrate, one ofthe electrodes and/or one of the additional transport layer(s), but notthe semiconductor, which therefore remains unstressed.

Examples of structurable substrates would be sheets or layers ofconventional polymers such as PET, PMMA, PC. These sheets can typicallyhave a layer thickness of between 10 and 1000 microns; the depth andperiod of the embossed periodic structure can be in the 10-1000 nmrange; the depth of aperiodic irregular embossed structures can be inthe 1-500 micron range.

Examples of planarizing layers having a high optical refractive indexwould be polyimides and/or inorganic-nanoparticle-(TiO₂)-filledpolymers.

1. An organic photovoltaic component comprising a substrate, a positiveelectrode, an organic semiconductor and a negative electrode, whereinsaid substrate and/or one or more additional transport layer(s) betweensaid electrode and said semiconductor layer is (are) structured.
 2. Theorganic photovoltaic component as defined in claim 1, wherein saidsubstrate is a flexible sheet that is structured.
 3. The organicphotovoltaic component of claim 1, wherein said substrate and/or anadditional layer above or below said semiconductor layer is structured.4. A method for structuring the semiconductor layer of a photovoltaiccomponent by preserving an existing structure of a lower layer to whichsaid semiconductor layer is applied.
 5. The method as defined in claim4, wherein said semiconductor layer planarizes the structure of saidlower layer(s).
 6. The method as defined in claim 4, wherein saidstructuring is effected by introducing an additional layer.
 7. Aphotovoltaic cell, comprising: a substrate; a first electrode supportedby the substrate; a second electrode; an organic semiconductor betweenthe positive and negative electrodes, wherein the substrate isstructured.
 8. The photovoltaic cell of claim 7, wherein the substrateis flexible.
 9. The photovoltaic cell of claim 7, wherein the firstelectrode is structured.
 10. The photovoltaic cell of claim 9, whereinthe first electrode is disposed on the substrate.
 11. The photovoltaiccell of claim 9, wherein the first electrode is a cathode.
 12. Thephotovoltaic cell of claim 7, further comprising a planarized layerbetween the substrate and the first electrode.
 13. The photovoltaic cellof claim 12, wherein the first electrode is disposed on the substrate.14. The photovoltaic cell of claim 7, further comprising a planarizedlayer between the organic semiconductor and the first electrode.
 15. Thephotovoltaic cell of claim 14, wherein the first electrode is disposedon the substrate.
 16. A photovoltaic cell, comprising: a substrate; afirst electrode supported by the substrate; a second electrode; anorganic semiconductor between the positive and negative electrodes,wherein the first electrode is structured.
 17. The photovoltaic cell ofclaim 16, wherein the substrate is not structured.
 18. The photovoltaiccell of claim 16, further comprising a first layer supported by thefirst electrode, the additional layer being structured.
 19. Thephotovoltaic cell of claim 18, further comprising a second layersupported by the first layer, the second layer being planarized.
 20. Thephotovoltaic cell of claim 16, wherein the substrate is flexible.